The present invention relates to a work-hardenable austenitic manganese (Hadfield type) steel having an elongation at rupture of 10 percent to 80 percent, and to a method for the production thereof.
Work-hardenable austenitic manganese steels have a wide range of application in the form of castings, forgings and rolled material. This wide use is due, in particular, to its high inherent ductility and satisfactory work-hardening ability. Uses range from castings for crushing hard materials to shell-proof objects. The valuable properties of manganese steel reside in the combination of the above-mentioned properties of work-hardening and ductility. Work-hardening
place whenever manganese steel is subjected to mechanical stress, for example, by shock or impact which converts the austenite in the surface layer partly to an epsilon-martensite. Measurements of work-hardening reveal an increase of between 200 and 550 in Brinell hardness. Thus, castings, forgings and the like increase in hardness during use, if they are subjected to mechanical stress. However, since such objects are also subjected to abrasion, the surface layer is constantly being removed, leaving austenite at the surface. This austenite is again converted by renewed mechanical stress. The alloy located below the surface layer is highly ductile, and manganese steels can therefore withstand high mechanical impact stress without any danger of rupture, even in the case of objects having thin walls.
In the case of objects to be made of manganese steel, it is essential that a preliminary mold or ingot-casting be produced in order to predetermine the properties of objects made therefrom. If the casting has an unduly coarse structure, the object will have low ductility. In the cases of large castings, it is known that grain-size varies over the cross-section. At the outside is a thin, relatively fine-grained edge zone, followed by a zone consisting of coarse columnar crystals, followed, in turn, by the globulitic structure at the center of the casting. Although the steel is essentially austenitic and work-hardenable over its entire cross-section, great differences arise in its mechanical properties, especially in its ductility, as a result of these structural differences.
In order to achieve the most uniform ductility possible over the entire cross-section, it has already been proposed that the casting temperature be kept as low as possible, for example at 1410.degree. C., since increasing super-cooling should cause the number of nuclei to grow and produce a finer grain-size. These low casting temperatures, however, cause major production problems. For instance, cold-shuts occur in the casting and the rheological properties of the molten metal are such that the mold is no longer accurately filled, especially at the edges. Furthermore, the molten metal solidifies, during casting, on the lining of the ladle, leading to ladle skulls or skins which must be removed and reprocessed. During actual casting, the plug may stick in the outlet, which means that pouring must be interrupted. It will easily be gathered from the foregoing that the economic disadvantages to be incurred for a non-reproducible refining of the grain are so serious that this low-temperature-casting process has not been able to gain acceptance.
Another method of refining the grain involves a specific heat-treatment, the casting being annealed for 8 to 12 hours at a temperature of between 500.degree. C. and 600.degree. C., whereby a large proportion of the austenite is converted into pearlite. This is followed by austenitizing-annealing at a temperature of between 970.degree. C. and 1110.degree. C. This double structural change is supposed to produce a finer grain, but it also causes the product to become extremely brittle during the heat-treatment, so that it ruptures without any deformation even under low mechanical stress. Another major disadvantage is that the process requires a considerable amount of energy.
For these reasons, attempts have already been made to achieve grain refining by adding further alloying elements, for example chromium, titanium, zirconium and nitrogen, in amounts of at least 0.1 percent or 0.2 percent by weight. Although at low casting temperatures, these additions or additives do refine the grain, they substantially impair mechanical properties, especially elongation and notch-impact strength.
Manganese steels (Hadfield type) usually have a carbon content of 0.7 percent to 1.7 percent by weight, with a manganese content of between 5 percent by weight and 18 percent by weight. A carbon:manganese ratio of between 1:4 and 1:14 is also essential if the properties of manganese steels are to be maintained. At lower ratios, austenitic steel is no longer present, the steel can no longer be work-hardened, and toughness is also impaired. At higher ratios, the austenite is too stable, again there is no work-hardening, and the desired properties are also not obtained.
A phosphorus content in excess of 0.1 percent by weight produces an extreme decline in toughness, so that, as is known, a particularly low phosphorus content must be sought.
ASTM A 128/64 describes four different kinds of manganese steel, with the carbon content varying between 0.7 percent by weight and 1.45 percent by weight and the manganese content between 11 percent by weight and 14 percent by weight. The carbon content is varied to alter the degree of work-hardening, and this may also be influenced by the addition of chromium in amounts of between 1.5 percent by weight and 2.5 percent by weight. Coarse carbide precipitations are to be avoided by adding up to 2.5 percent by weight of molybdenum. An addition of up to 4.0 percent by weight of nickel is intended to stabilize the austenite, thus preventing the formation of pearlite in thick-walled castings.
Also known is manganese steel containing about 5 percent by weight of manganese. Although such steels have little toughness, they have high resistance to wear.